In the event of sulphurous acid being used, it would be necessary to import cylinders of sulphur dioxide from which the solution could be prepared in factories each day. There would be no insurmountable difficulty in this, as it is only necessary to pass the gas through a series of closed vessels containing water. Enough solution could be prepared at one time for three or four days, but preferably the solutions should be as fresh as possible. Altogether there would seem to be possibilities in the use of sulphurous acid for preparing pale crepe rubbers, providing the cost is within comparable limits with the commoner coagulants at present in use, and that no adverse effect on the rubber can be shown to result. If the cost did not exceed the combined cost of acetic acid and sodium bisulphite, the employment of sulphurous acid solution might be worthy of consideration. There is one drawback to the use of sulphurous acid solution, and that lies in the proximity of the limits of the quantities necessary for coagulation and that which is in excess, and prevents coagulation. Thus, with ordinary field latex having about 20 per cent. dry rubber content, the minimum necessary for coagulation per 100 c.c. of latex is about 8 c.c. of a 1 per cent. solution. The maximum quantity possible for use is about 15 c.c. of a 1 per cent, solution, so that great care would have to be exercised in avoiding an excess of coagulant, otherwise coagulation would be effectually prevented.
It is believed that the preparation of rubber by this method is the subject of a patent secured by Messrs. Boake, Roberts, and Co., London.
Sugars.—Coagulation may be effected by the addition of small quantities of sugars.[26] These are assumed to be effective by fermentative conversion into lactic and acetic acids. The presence of lactic acid is supposed to have a twofold effect:
(a) As a direct coagulant.
(b) In its action upon certain organisms which, in the ordinary course of events, would delay or prevent coagulation. Although work on an experimental scale has been done, as far as we know no practical application has been made of the employment of sugars as coagulating agents.
[26] “Preparation and Vulcanisation of Plantation Para Rubber” (Eaton, Grantham, and Day), Bulletin No. 27, F.M.S. Department of Agriculture; Gorter and Swart, Bulletin No. 6, West Java Expt. Station.
Various Salts.—Of experimental interest only it may be recorded that coagulation has been effected by means of various chemical “salts“—e.g., calcium chloride, barium chloride, magnesium chloride, sodium chloride, aluminium sulphate, magnesium sulphate, sodium sulphate, etc. None of these has been found to have any practical application, except, perhaps, calcium chloride, which is used in small quantity as an accelerating agent in a special process of anaerobic coagulation, which will receive mention in the [following chapter].
At one period during the War and the dearth of acetic acid, it was found that there were available in England large supplies of the acid sulphate of sodium (sodium hydrogen sulphate), which proved to be an effective coagulant. Experimental work gave satisfactory results, but no practical application resulted when supplies of acetic acid were again obtainable.
Various Proprietary Compounds.—We have seen many proprietary coagulants advertised and pass into the limbo of forgotten things. They can generally be divided into two classes. The first embraces those founded upon a woefully incomplete knowledge of requirements. The second covers those which meet requirements, but for which exaggerated claims are made and excessive prices charged.
As as instance of a substance which fell under both classifications might be mentioned the case of “Coagulatex.” Pretentious claims were made, and it was emphasised that the liquid contained no vegetable acids. Acetic and formic acids might be quoted as examples of vegetable acids, and as these have been shown to be the most satisfactory coagulants now employed one fails to imagine where lay the value of the guarantee given by the advertisers of “Coagulatex.”